A level-up shifter is an essential element for IO design which handles signals crossing from the core power supply domain to the IO power supply domain, with the IO supply higher than the core supply. For the full-swing GHz high speed signals, a level-up shifter which can handle rail to rail signal level and still preserve duty cycle fidelity is in high demand. More importantly, as power dissipation becomes an important factor in IC design, it is critical to reduce DC current paths. Since a level-up shifter needs to transfer signals from one power domain to another, short-circuit currents may still be present when driving a static DC level if not designed carefully, particularly as the difference between core and IO supply grows larger.